Development of Ketone-Based Brominating Agents (KBA) for the Practical Asymmetric α-Bromination of Aldehydes Catalyzed by Tritylpyrrolidine

Stereoselective Electrophilic Chlorination of β,β-Disubstituted Enesulfinamides with Chloramine-T: Asymmetric Synthesis of Acyclic α,α-Disubstituted α-Chlorinated Carbonyl Surrogates

Enamine and iminium ion-mediated asymmetric organocatalysis was not successful in achieving highly stereoselective α-chlorination of acyclic α,α-disubstituted carbonyls. To address this limitation, an alternative method was developed, which involved the use of geometry-defined persubstituted enesulfinamides to intercept the electrophilic chlorinating reagent. This approach enables the asymmetric construction of challenging acyclic α,α-disubstituted α-chlorinated ketimines with a high degree of stereoselectivity. The use of chloramine-T, a cost-effective and stable chlorine source rarely utilized in asymmetric electrophilic chlorination, plays a crucial role in achieving superior stereocontrol.

One-pot furan synthesis through diethylzinc-mediated coupling reaction between two α-bromocarbonyl compounds

Polysubstituted furans were synthesized in one-pot through the Et2Zn-mediated coupling reaction between dibromoketones and monobromo carbonyl compounds and the subsequent β-elimination with bromoacetyl bromide. Polysubstituted pyrroles were also prepared in one-pot by addition of primary amines after the coupling reaction.

Construction of three contiguous stereocenters through amine-catalyzed asymmetric aldol reactions

Three contiguous stereocenters were constructed by an amino acid-catalyzed asymmetric aldol reaction of α-siloxyketones with racemizable α-haloaldehydes via dynamic kinetic resolution. One-pot catalytic asymmetric synthesis of the highly functionalized products could also be accomplished by the α-bromination of simple aldehydes and the subsequent asymmetric aldol reaction.

Oxido-Molybdenum(V) Corroles as Robust Catalysts for Oxidative Bromination and Selective Epoxidation Reactions in Aqueous Media under Mild Conditions

Two new meso-substituted oxido-molybdenum corroles were synthesized and characterized by various spectroscopic techniques. In the thermogram, MoO[TTC] (1) exhibited excellent thermal stability up to 491 °C while MoO[TNPC] (2) exhibited good stability up to 318 °C. The oxidation states of the molybdenum(V) were verified by electron paramagnetic resonance (EPR) spectroscopy and exhibited an axial compression with d_xy¹ configuration. Oxido-molybdenum(V) complexes were utilized for the selective epoxidation of various olefins with high TOF values (2066-3287 h^-1) in good yields in a CH₃CN/H₂O (3:2, v/v) mixture in the presence of hydrogen peroxide as a green oxidant and NaHCO₃ as a promoter. The oxidative bromination catalytic activity of oxido-molybdenum(V) complexes in an aqueous medium has been reported for the first time. Surprisingly, MoO[TNPC] (2) biomimics of the vanadium bromoperoxidase (VBPO) enzyme activity exhibited remarkably high TOF values (36 988-61 646 h^-1) for the selective oxidative bromination of p-cresol and other phenol derivatives. Catalyst MoO[TNPC] (2) exhibited higher TOF values and better catalytic activity than catalyst MoO[TTC] (1) due to the presence of electron-withdrawing nitro groups evident from cyclic voltammetric studies.

Diethylzinc-Mediated Cross-Coupling Reactions between Dibromoketones and Monobromo Carbonyl Compounds

A novel route to synthesize 1,4-dicarbonyl compounds is described. α,α-Dibromoketones generate zinc enolates through a diethylzinc-mediated halogen-metal exchange and react with α-bromocarbonyl compounds to furnish 1,4-dicarbonyl compounds via a second generation of zinc enolates. This cross-coupling reaction is enabled by the chemoselective formation of zinc enolates from α,α-dibromoketones in the presence of α-bromocarbonyl compounds. Chiral 1,4-dicarbonyl compounds can be obtained via the enantioselective bromination of aldehydes using a chiral secondary amine catalyst and a subsequent cross-coupling reaction between the resulting chiral α-bromoaldehydes and α,α-dibromoacetophenones.

α-Thianthrenium Carbonyl Species: The Equivalent of an α-Carbonyl Carbocation

Here we report an α-thianthrenium carbonyl species, as the equivalent of an α-carbonyl carbocation, which is generated by the radical conjugate addition of a trifluoromethyl thianthrenium salt to Michael acceptors. The reactivity allows for the synthesis of C^α -tetrasubstituted α- and β-amino acid analogues via a Ritter reaction by addition of acetonitrile. Addition of hydroxide, methoxide, and even fluoride can afford α-heteroatom substituted α-phenylpropanoates.

Organocatalytic Enantioselective α-Bromination of Aldehydes with N-Bromosuccinimide

Despite the wealth of existing organocatalytic, enantioselective transformations, the α-bromination of aldehydes remains a challenging reaction. The four examples reported to date require expensive, inconvenient brominating agents to achieve the desired products in excellent yields and enantioselectivities. The preferred brominating agent, N-bromosuccinimide (NBS), has been repeatedly discarded for these reactions because it results in low yields and relatively poor enantioselectivities. We describe a methodology that uses NBS and performs excellently with low catalyst loadings, short reaction times, and mild temperatures.

Synthesis of enantioenriched α-heteroatom functionalised aldehydes by chiral organocatalysis and their synthetic applications

Asymmetric organocatalysis is a versatile method for the enantioselective α-functionalisation of aldehydes. The synthetic scope for chiral α-heteroatom substituted aldehydes is examined including their applications in synthesis.